WL#1879: Decreasing size of operation records

Currently the operation records are fairly "spacy".
An optimisation is to decrease the size of them
significantly. In DBTC as an example it should be possible to decrease
to 8 bytes per operation after the prepare phase.
Also TC take-over could use separate records which are smaller in size
and specialised for TC take-over.
5.0 or 5.1 feature